Method of making manganese-aluminum-carbon ternary alloys for permanent magnets

ABSTRACT

TERNARY ALLOYS CONSISTING ESSENTIALLY OF, BY WEIGHT, (A) 67 TO 69% MN. 29 TO 32.0% AL, AND 0.3 TO 3.0% C, AND (B) 70.0 TO 72.5% MN, 26.5 TO 29.0% AL AND 0.5 TO 2.5% C, ARE PREPARED BY HEATING THE SELECTED ALLOY COMPOSITION TO A TEMPERATURE OF ABOUT 1380*C. TO FORM A MELT, CASTING THE MELT IN A MOLD TO FORM AN INGOT, QUENCHING THE INGOT FROM A TEMPERATURE OF 880* TO 1250*C. AND THEN ISOTHERMALLY TEMPERING THE QUENCHED INGOT AT A TEMPERATURE OF 380* TO 700*C. THE THUS HEAT-TREATED ALLOYS ARE SUITABLE FOR USE AS PERMANENT MAGNETS WITH MAGNETIC PROPERTIES SUPERIOR TO THOSE OF BINARY ALLOYS OF SIMILAR MANGANESE AND ALUMINUM CONTENTS.

1973 HIROSHI YAMAMOTO 3,730,784

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United States Patent 3,730,784 METHOD OF MAKING MANGANESE-ALUMINUM-CARBON TERNARY ALLOYS FOR PERMANENT MAGNETS Hiroshi Yamamoto, Osaka,Japan, assignor to Matsushita Electric Industrial Co., Ltd.,Kadorna-shi, Osaka, Japan Continuation of application Ser. No. 429,260,Feb. 1, 1965. This application July 28, 1969, Ser. No. 850,307 Claimspriority, application Japan, Feb. 1, 1964, 39/5,653; Mar. 3, 1964, 39/12,679 Int. Cl. H011? 1/04 US. Cl. 148-401 10 Claims ABSTRACT OF THEDISCLOSURE Ternary alloys consisting essentially of, by weight, (a) 67to 69% Mn, 29 to 32.0% Al, and 0.3 to 3.0% C, and (b) 70.0 to 72.5% Mn,26.5 to 29.0% Al and 0.5 to 2.5% C, are prepared by heating the selectedalloy composition to a temperature of about 1380 C. to form a melt,casting the melt in a mold to form an ingot, quenching the ingot from atemperature of 880 to 1250 C. and then isothermally tempering thequenched ingot at a temperature of 380 to 700 C. The thus heat-treatedalloys are suitable for use as permanent magnets with magneticproperties superior to those of binary alloys of similar manganese andaluminum contents.

This application is a continuation of copending application Ser. No.429,260, filed Feb. 11, 1965, and now abandoned.

The present invention relates to an entirely novel material forpermanent magnets and more particularly to manganese-aluminum-carbonternary alloys in which a suitable amount of carbon is added positivelyas a new and third element to the already known magnetic material ofmanganese-aluminum binary alloys.

The primary object of the present invention is to provide a method formaking manganese-aluminum-carbon ternary alloys for permanent magnets,whose residual induction Br, coercive force H maximum energy productBI-I saturation magnetization 411-1 and others, are higher than those ofthe prior magnetic materials of manganese-aluminum binary alloys.

According to a prior method for obtaining permanent magnets ofmanganese-aluminum binary alloys, when an alloy consisting of about 72%by weight manganese and about 28% by Weight aluminum is subjected to asuitable heat treatment, it transforms from the high temperaturehexagonal epsilon phase to tetragonal metastable phase. Themanganese-aluminum alloys thus subjected to the transformation becomeferromagnetic, but their magnetic properties in respect of, for example,BH is quite poor or only of the order of 0.6)( G-oe. Therefore, it hasbeen diflicult to use the manganese-aluminum binary alloys for practicalapplication as permanent magnets.

The inventors have discovered that alloys in which carbon is addedpurposely as a third element to such manganese-aluminum alloys are soexcellent that they have improved magnetic properties, have a highantioxidation property, have a widened composition range showingexcellent magnetic properties and can be subjected to heat treatmentsunder conditions which are not so severe as compared with priorconditions.

According to the present invention, there is also provided a method formaking rnanganese-aluminum-carbon ternary alloys for extremely powerfulpermanent magnets, comprising the steps of preparing a mixtureessentially consisting of from 67.0 to 69.0% by weight metallicmanganese, from 29.0 to 32.0% by weight metallic alu- 3,730,784 PatentedMay 1, 1973 minum and from 0.3 to 3% by weight carbon, or consisting offrom 70.0 to 72.5% by weight metallic Mn, from 26.5 to 29.0% by weightAl and from 0.5 to 2.5 by weight C, heating to melt the mixture at atemperature range of 1260 to 1500 C. in at least one atmosphere selectedfrom the atmosphere group of inert gas, reducing gas and vacuum, castingthe melt into a mold to obtain an ingot, quenching the ingot from atemperature range of 880 to 1250 C., and then tempering the ingot at atemperature range of 380 to 760 for a predetermined time.

Other objects and particularities of the present invention will becomemore obvious as the description proceeds.

In the accompanying drawings:

FIG. 1 is a composition diagram of manganese-aluminum-carbon ternarysystem in which the portion surrounded by thick line shows a preferredcomposition range of the alloys according to the present invention; and

FIGS. 2 to 6 are graphic representations of equivalent curves ofsaturation magnetization 41rI residual induction Br, coercive force He,intrinsic coercive force Hc, and maximum energy product EH of theinventive alloys as drawn on the composition diagram thereof,respectively.

Metallic manganese, metallic aluminum and carbon are weighed in variouspercentages to obtain mixtures of the three elements of variouscompositions. The mixtures are charged into respective crucibles and areheated to melt in one or more of atmospheres selected from the group ofinert gas, reducing gas and vacuum. Melting temperatures for themixtures vary depending on their compositions, but lie within a range of1260 to 1500 C. Preferred melting temperature is about 1380 C. and atthis temperature the three elements are effectively alloyed. It ispreferred that the mixtures are heated up to about 800 C. in a vacuumand in a temperature range thereabove heated in an argon atmosphere.Then the melts are cast into suitable molds or are cooled in thecrucibles to obtain ingots of predetermined size. In order to homogenizethe alloys, the ingots may be subjected to forging, solution treatmentor any other treatment, but this step is not necessarily required.

Table 1 below shows chemical compositions, as determined by chemicalanalysis, of thirty-three specimens selected from the ingots of themanganese-aluminum-carbon ternary alloys thus obtained. FIG. 1 shows acomposition diagram of the manganese-aluminum-carbon ternary alloyswhich are included in. the composition range within the lineA-G-H-I-E-F.

TABLE 1 Percent by weight; Specimen number Mn Al 0 60. 2 30. 5 0. 3 70.0 29. 6 0. 4 71. 4 28. 0 0. 6 71. 8 27. 4 0. 8 72. 5 26. 5 1. 0 73. 525. 5 1. 0 69. 0 28. 1 2. 0 68. 4 20. 3 2. 3 69. 5 27. 5 3. O 70. 2 28.3 1. 5 70. 8 27. 8 1. 4 71. 2 27. 2 1. 6 72. 1 26. 3 1. 6 70. 5 26. 4 3.1 70. 3 25. 7 4. 0 71. 6 25. 0 3. 4 69. 0 26. 5 4. 5 72. 5 24. 5 3. 073. 7 26. 2 0. 1 72. 1 27. 9 0. 02 67. 9 31. 8 0. 3 66. O 33. 7 0. 3 65.1 34. 0 0. 9 67. 3 31. 6 1. 1 68. 9 B1. 0 0. l.

TAB LE I-C'ontinued Percent by weight Some of these alloys showferromagnetic properties as they are molten and as cast. No. 4 specimen,for example, has magnetic properties of the order of Br.=2000 G, Hc=500e. and BH,,,,,,,=0.4 G-oe. Since however these values are stillinsuflicient for a permanent magnet, all the specimens are subjected tothe following heat treatments. In a first step, the specimens aresubjected to water or oil quenching from a temperature of 1100 C. Thequenching temperature may desirably suitably be varied depending on thealloy compositions, but a quenching temperature range of 880 C. to 1250C. is sufficient. Then, the quenched alloys are subjected to a secondheat treatment or isothermal tempering in a temperature range of 380 C.to 760 C. for a suitable time of the order of from several minutes toseveral hundred hours. By these heat treatments, those alloys which didnot show any ferromagnetic properties as they were cast becomeferromagnetic, and those which already showed ferromagnetic propertiesas they cast show further improved magnetic properties.

The purpose of the two heat treatments is to cause in the ternary alloysthe transformation similar to that caused in the manganese-aluminumbinary alloys. But the difference between there is that, in the ternaryalloys of the present invention including therein carbon, allowableranges of temperature and time in both of the first step and the secondstep heat treatments are far wider than in the case of the binaryalloys. Temperature and time settings for the second heat treatment varydepending on the alloy composition, and magnetic properties obtainedalso greatly vary by a combination of temperature and time. Conditionsfor obtaining best magnetic properties with respect to the respectivealloys are not common to all cases. Though it is difiicult to set upcomprehensive conditions for all of the ternary alloys, the optimumconditions may, for example, be such as are shown in Table 2. As seenfrom Table 2, the tempering temperature for the ternary alloys is 500 to650 C., preferably.

TABLE 2 Temperature C.

Specimen number Time (hr.)

It will be seen from the above table that No. 3 alloy, for example,shows best magnetic properties when tempered for 3 hours at 600 C. orfor 8 hours at 540 C. In case of No. 21 alloy, it shows a maximum energyproduct BH of 1.05 X 10 G-oe. When quenched from 1150 C. and temperedfor 6 hours at 500 C., and shows a maximum energy product BH of the samevalue when tempered for 35 minutes at 620 C. No. 21 alloy how ever showspoor magnetic properties when tempered for 35 minutes at 500 C. and ithas a poor BH value of 0.4 10 G-oe. Even when tempered for 6 hours at500 C., No. 29 alloy shows a poor BH value of 0.2 10 G-oe. Thejust-mentioned alloy however shows a high BH value of 1.18 10. G-oe.under diiferent heat treatment conditions. In General, it seems thatbest results can be obtained at a higher tempering temperature and alonger time in case of the manganese rich alloys and at a lowertempering temperature and a shorter time in case of the manganese pooralloys.

As described above, there are optimum tempering temperature and time foreach of said alloy compositions, and it is diflicult to set up definitetemperature and time settings common to all of the alloy compositions.Therfore, it is to be understood that magnetic properties of the alloysof various compositions hereunder described represent the values wheneach alloy is subjected to a heat treatment by which it shows the mostexcellent properties.

Magnetic properties of the ternary alloys are as tabulated in Table 3and, when shown in the form of equivalent curves on the ternarycomposition diagram are as depicted in FIGS. 2 to 6.

TABLE 3 Specimen 41rIs Br Ho BHmnx. Number (G) (G) (0e.) (0e.) (X 10 G0e.)

Non-magnetlc Non-magnetic 900 2, 050 0. 61 1, 500 1,000 1, 500 0. 51 2,600 1, 250 1, 400 1.05 1, 800 600 900 0.40 1, 900 750 1, 000 0.60 2, 7001, 250 1, 300 1. 26 2, 400 1,200 1, 400 0.80 2, 550 1, 200 250 1. 21 1,300 1, 050 1, 0.80

N onmagnetie 1,750 1,200 1,300 1.18 950 850 1, 000 0.42

Non-magnetic 900 800 850 0. 32

Non-magnetic From the above table, it can roughly be concluded that thealloys having a low manganese content have high values of 41r Is and Brand low values of He and Hc, while those having a high manganese contenthave low values of 41r Is and Br and high values of He and HC. However,values of BH obtained by a relation between Br and He show fluctuationof extremely complicated nature and it is impossible to find out adefinite tendency thereof. Roughly, those alloy compositions in cludingabout 1 to 2%! by weight carbon show good magnetic properties. In thiscase too, excellency of the magnetic properties is greatly influenced bythe amount of maganese and aluminum. As far as BH is concerned, it seemsthat BH is greatly influenced by the percentages of manganese, aluminumand carbon in the ternary alloys. Those alloys falling within acomposition range of from 67 to 69% by weight manganese, from 29 to 32%by weight aluminum and from 0.3 to 3.0% by weight carbon and acomposition range of from 70 to 72.5% by weight manganese, from 26.5 to29% by weight aluminum and from 0.5 to 2.5% by weight carbon showespecially excellent magnetic properties and they have a BH value of BHZLOX l G-oe.

Some of the manganese-aluminum-carbon ternary alloys consisting of from65.0 to 69.0% by weight manganese, from 31.0 to 34.0% by Weightaluminum, and less than 1.0% by Weight carbon, that is, the alloyshaving a composition falling within a portion surrounded by the lineA-B-C-F in FIG. 1, have a BH value of the order of 1.05 10 G-oe. Some ofthe manganese-aluminumcarbon ternary alloys consisting of from 65.0 to68.0% by weight manganese, from 28.0 to 34.0% by weight aluminum andfrom 1.0 to 4.0% by weight carbon, that is, the alloys having acomposition falling within a portion surrounded by the line C-D-E-F,have a BH value of the order of 1.26X 10 G-oe. Further, some of themanganese-aluminum-carbon ternary alloys consisting of from 68.0 to74.0% by weight manganese, from 25.0 to 31.0% by weight aluminum andless than 4.0% by weight carbon, that is, the alloys having acomposition falling within a portion surrounded by the line B-G-H-I-D-C,have a BH value of the order of 1.83 10 G-oe.

From the foregoing description, it will be known that themanganese-aluminum-carbon ternary alloys according to the presentinvention have a very wide composition range in which they show a BHvalue of BH ZLOX l0 G-oe. In contrast, the manganese-aluminum binaryalloys which generally have a BH value of the order of 0.5 to 0.6 10G-oe., have a narrow composition range showing ferromagnetic propertiesand have a narrow allowable range of heat treatment for the purpose ofobtaining ferromagnetic properties. It is quite apparent from the aboveexperiments that the alloys obtained by the above-mentioned processprovide a BH value of the order of BH gOJ X 10 G-oe. over a widecomposition range of from 65.0 to 74.0% by weight manganese, from 25.0to 34.0% by weight aluminum and less than 4.0% by weight carbon, and canbe heat-treated under conditions which are not so severe compared withthe manganese-aluminum binary alloys. Thus, it Will be understood thatthe addition of carbon exhibits such a great elfect.

Carbon may be added in any amount less than 4.0% by weight excludingzero since addition of carbon in a very small amount, for example, 0.02%by weight can efiectively improve the magnetic properties. Further, themagnetic properties of the inventive alloy would not be affected by theinclusion therein of impurity elements, other than manganese, aluminumand carbon, which may commonly be found in ordinary alloys. No variationin the magnetic properties was observed even with inclusion of certainimpurity elements in an amount up to 2% by weight.

What is claimed is:

1. A method of making manganese-aluminum-carbon ternary alloys for apermanent magnet, comprising the steps of 1) selecting an alloycomposition which consists essentially of (a) 67.0 to 69.0% manganese byweight, 29.0 to 32.0% aluminum by weight and 0.3 to 3.0% carbon byweight or (b) 70.0 to 72.5 manganese by Weight, 26.5 to 29.0% aluminumby weight and 0.5 to 2.5 carbon by weight, (2) heating the selectedalloy composition at a temperature of about 1380 C. to form a melt, (3)casting the melt into a mold to obtain an ingot, (4) quenching the ingotfrom a temperature of 880 to 1250 C., and (5) then isothermal temperingthe ingot at a temperature of 380 to 760 C.

2. The method according to claim 1 in which the alloy compositionconsists essentially of 67.0 to 69.0% manganese by Weight, 29 to 32%)aluminum by Weight and 0.3 to 3% carbon by Weight.

3. The method according to claim 1 in which the alloy compositionconsists essentially of 70.0 to 72.5% manganese by Weight, 26.5 to 29.0%aluminum by weight and 0.5 to 2.5% carbon by weight.

4. The method according to claim 1 in which heating step (2) is carriedout in an atmosphere selected from the group consisting of an inert gas,a reducing gas and a vacuum.

5. The method according to claim 1 wherein tempering step (4) is carriedout for a period of 1.0 to 6.0 hours.

6. The method according to claim 1 in which heating step (2) is carriedout in two stages, first under vacuum from room temperature to about 800C., then in an argon atmosphere to a temperature of about 1380 C.

7. The method according to claim 1 in which the ingot is quenched inoil.

8. The method according to claim 1 in which the ingot is quenched inwater.

9. The method according to claim 2 in which the ingot is quenched from atemperature of 1100 C. in step (3) and the quenched ingot is thentempered at a temperature of 500 to 650 C. in step (4).

10. The method according to claim 3 in which the ingot is quenched froma temperature of about 1100 C. in step (3) and the quenched ingot isthen tempered at a temperature of 500 to 650 C. in step (4).

References Cited UNITED STATES PATENTS 1,750,751 3/1930 Geyer 962,797,995 7/ 1957 Morgan 75134.9 3,116,181 12/1963 Hokkeling et al.148-3157 3,236,636 2/1966 Finkl 75--96 3,266,954 8/1966 Hokkeling et a1.1483l.57

L. DEWAYNE RUTLEDGE, Primary Examiner G. K. WHITE, Assistant ExaminerU.S. Cl. X.R.

